The present invention relates to wide-band antennas of the plane spiral type which have one or two conductive arms arranged spirally in a plane close to a reflector. Such antennas are widely known for their ability to operate with a wide band width. In a given frequency band, they possess a polar diagram having a constant 3dB aperture, a constant gain and a low level of ellipticality. These characteristics are achieved without any great difficulty in a frequency band in which the ratio between the upper frequency and the lower frequency is 3:1. When the ratio greatly exceeds 3:1 two main defects become apparent:
The first defect is due to the presence of the reflector, which in the case of the upper frequencies, needs to be situated at a distance from the spiral which is less than a quarter of the wave-length (.lambda./4). This means that for the lower frequencies the distance in question is no more than .lambda./12 when the frequency ratio is 3:1 and .lambda./24 when it is 6:1. This being so, at low frequencies the radiation resistance is low and efficiency falls, resulting in a loss of gain.
The second defect is due to the fact that only a relatively restricted area of the antenna contributes to its radiation at any given frequency. At low frequencies this area is situated towards the periphery of the antenna. As the frequency increases this radiation area moves nearer the centre of the spiral. In what follows this area will be known as the main radiation area since at high frequencies one or other interference areas appear, first near the periphery of the antenna and then increasingly near the centre as the frequency continues to rise. The relative phases of the currents in the main and interference areas alter rapidly as a function of frequency. The polar diagram is then no longer in the form of a body of revolution about the axis of the antenna but takes on an elliptical cross-section. This results in sudden alterations in the width of the 3dB polar diagram and the axis along which radiation is at a maximum may, in the case of polar diagrams plotted in planes which do not pass through the axis perpendicular to the plane of the antenna, diverge by up to 10 to 15.degree. from its normal position.
Known solutions which enable the effect of radiation from the interference areas to be reduced consist in attenuating the currents along the spiral by the effect of series or parallel losses.
Series losses may be brought about by reducing the width of the conductors towards the end of the spiral, which results in increased losses per unit length in the conductor, or by reducing the pitch of the spiral which results in an increase in the length of the conductors.
Parallel losses are brought about by using a lossloaded dielectric support for the conductors.
These solutions are not satisfactory in that they involve a loss of gain in the antenna over the whole of the operating band and in particular at the low frequencies where the nearness of the reflector is already having an unfavourable effect.
The solutions proposed in the present application are not subject to these drawbacks.